Vinyl aromatic resins, e.g., polystyrene, have been found to be useful in thermoplastic molding compositions. Vinyl aromatic resins have poor heat distortion and impact resistance, however, and attempts have been made to upgrade these properties. One approach has been to modify the vinyl aromatic resins by copolymerizing these materials with .alpha.,.beta.-unsaturated cyclic anhydrides, to form copolymers such as poly(styrene-maleic anhydride). Although improvements in heat resistance and solvent resistance are provided, the resulting copolymers are somewhat brittle, and they do not have good resistance to impact.
Various attempts have been made to improve the impact resistance of copolymers of vinyl aromatic resins and .alpha.,.beta.-unsaturated cyclic anhydrides. For instance, these copolymers have been blended with nitrile rubbers. Blends of nitrile rubber and styrene-maleic anhydride copolymers are disclosed in U.S. Pat. Nos. 2,914,505 and 3,641,212. With some of these compositions, however, the components are not compatible, and the compositions are difficult to prepare.
The following commonly assigned copending applications disclose proposals to solve the problems stated above. Lee and Abolins, Ser. No. 477,435, filed June 7, 1974, who employ block copolymers or graft copolymers in combination with the vinyl aromatic/unsaturated cyclic anhydride copolymers; Lee, Ser. No. 671,569, filed Mar. 29, 1976, who discloses block copolymers with rubber-modified vinyl aromatic/unsaturated cyclic anhydride copolymers; Abolins and Lee, Ser. No. 671,341, who disclose polyphenylene ether resins with vinyl aromatic/unsaturated cyclic anhydride copolymers; and Haaf and Lee, Ser. No. 693,895, filed June 8, 1976, who disclose radial teleblock copolymers in combination with vinyl aromatic/unsaturated cyclic anhydride copolymers, optionally rubber modified. The applications are incorporated herein by reference.
It has now been surprisingly discovered that copolymers of a vinyl aromatic compound and an .alpha.,.beta.-unsaturated cyclic anhydride can be combined with an all acrylic emulsion graft copolymer to form compositions which can be molded to articles having excellent mechanical properties, including good impact strength, tensile yield and elongation, especially when combined with a polyphenylene ether resin. It has further been discovered that such an all acrylic emulsion graft copolymer can be used to enhance the important properties of rubber modified vinyl aromatic/unsaturated cyclic anhydride copolymers), with and without the addition of a polyphenylene ether resin.
It has been further discovered that the toughness of both unmodified and rubber-modified vinyl aromatic/unsaturated cyclic anhydride copolymers can be remarkably enhanced by combination with a vinyl aromatic, acrylonitrile-diene graft copolymer. The results are surprising in view of earlier work with an acrylic-vinyl aromatic-diene rubber graft copolymer.
Still a further discovery has been made in which compositions comprising a vinyl aromatic/unsaturated cyclic anhydride copolymer, a polyphenylene ether resin, and a graft copolymer resin, a rubber modified vinyl aromatic resin, or a segmented copolyester resin are vastly improved in ductile impact properties by adding a small amount normally liquid diene oligomer.
A further broad discovery resides in finding that lower than expected levels of polyphenylene ether resin can be employed in combination with vinyl aromatic/unsaturated cyclic anhydride copolymers, than would have been expected from work with vinyl aromatic homopolymers and graft copolymers, thus permitting retention of heat distortion temperatures at higher predetermined levels.